Block copolymers of vinyl aromatic hydrocarbons, such as styrene, and conjugated dienes, such as isoprene and butadiene, are well known. Generally, these are made by polymerizing a polymer block of one of the monomers, for instance, styrene, until all of the monomer is gone, introducing a second monomer and polymerizing it until all of it is gone, and so on. This process would produce block copolymers which have distinct homopolymer blocks. It is also possible to polymerize more than one monomer together to produce random copolymer blocks within the block copolymer.
In some situations, it has been found advantageous to create a random copolymer block within the block copolymer which is "tapered." A tapered block is a randomly copolymerized block wherein the conditions of polymerization are managed such that the concentration of one of the mers in the block copolymer is highest at one end of the polymer block and generally decreases in the direction of the other end of the polymer block so that the concentration of other mer is greatest at the other end. In these tapered block copolymers, the trend is as described previously but the distribution of the mers is still random.
It has been found that it would be advantageous to be able to produce a tapered polymer block in which the random nature of the distribution of the mers in the polymer block is dramatically decreased in comparison to the prior art tapered block copolymers. The present invention provides a process which allows the production of block copolymers having tapered blocks of styrene and isoprene having a specific taper structure which consists of small segments of variable size of styrene monomer and isoprene monomer wherein the concentration of the isoprene mers at one end of the polymer is high and gradually decreases in the direction of the other end of the polymer block but wherein the random distribution of the mers in the block is substantially decreased.